Natural resources form an important part of the economic development of a country. Although our dependence on these resources contribute to economic benefits, when close attention is paid certain actions become a major threat to the environment we live in. Despite the benefits we reap out of the resources, it is important to look into the ill effects of it as well.

The advent of Prosopis juliflora in the early 1970s is a classic example of this condition. It was introduced in several countries as an attempt to limit the risk of deforestation and shortage of fuelwood. Interestingly with time, it turned out to be a noxious invasive weed posing a major threat to the environment.

Native to the South America and Carribean, the plant gained a lot of importance in India in the initial years for its economic benefits. As its invasive nature aggravated, the negative impacts of the species became more transparent. In 2004, the International Union for Nature Conservation (IUCN) recognised Prosopis as one of the 100 most unwanted species in the world.

Two main factors called for the need for eradication of the species: the deep penetrating roots caused serious imbalance in the water table, drying up the water bodies in the surroundings. Therefore their presence, mainly in the arid or semi arid regions, created an unstable environment for other inhabitant species. Another problem here was their ability to spread faster and to thrive despite dry and drought conditions. They form impenetrable thickets suppressing the growth of the indigenous species, also colonising the agricultural lands to a large extent in most of the cases studied so far.

For instance, the curse of Prosopis was quite evident in the case of Point Calimere Wildlife and Bird Sanctuary. The sanctuary gained the status of a Ramsar site in the year 2002, recording the largest congregation of migratory waterbirds in the country exceeding a count of 100,000. However, the dwindling population of Blackbuck and the need to conserve it was also one of the main motives behind its creation in 1967. According to March 2005 census, the population of Black buck had grown to around 1450, harbouring the largest population of blackbucks in south India.

The Mesquite-blackbuck conflict

Although Prosopis invasion has created several damages to the sanctuary in various ways, one problem that needs major attention is the destruction it caused to the open grasslands of the sanctuary. Comparative studies conducted using the LANDSAT ETM imageries of the year 1999 revealed an increase in the growth of Prosopis mainly in the Tropical Dry Evergreen Forests and the open grasslands.

Cattle grazing have already been established as a major issue within the sanctuary. It was found that the food consumption of one cattle head is equal to that of the entire blackbuck population of the sanctuary. In addition to this, the Prosopis invasion of the open grasslands poses a major threat to the survival of the blackbucks. Other than the blackbucks, animals like the spotted deer, Black-naped hare, Small Indian Civet and the Indian star tortoise also frequent the grasslands. Therefore, the removal of Prosopis in the open grasslands becomes crucial because it serves as a major feeding habitat for the blackbucks and various other species in the sanctuary as well.

Factors contributing to Prosopis invasion

As the sanctuary is surrounded by the sea and backwater swamps on all the four sides, the areas around it became one of the largest salt producing regions in the country. The overflow of bittern from the nearby salt factory has largely led to the encroachment of the northern banks of Muniappan lake by Prosopis. Interestingly the inverse effect of frugivores mainly Bonnet monkeys and Feral Ponies as seed dispersal agents can be noticed in the case of Prosopis as they contribute vastly for their spread.

The need for eradication

It is important to understand that the substitution of natural vegetation with foreign ones can prove fatal to the existing bio diversity of the forest. The invasion of Prosopis today can be compared to the loss of forest area and disappearance of animals in the sanctuary in the late 40s. This condition occurred as a consequence of the government’s experimental measure to grow Casuarina within the forest. A striking evidence for the ill effects of Prosopis was established through plot studies conducted by Dr. Rauf Ali of FERAL, Pondicherry in 2005. It revealed that the number of trees were 63% higher in the plots without Prosopis. As the Net Primary Productivity (NPP) of Prosopis is very low, it creates a less productive environment for the entire sanctuary. On one hand as it contributes to the decrease in open grasslands, on the other hand it is enabling the spread of Tropical Deciduous Evergreen Forests (TDEF) at a faster rate. This phenomenon again has dire consequences of its own.

All the studies continuously remind us about one important fact. It is the necessity to preserve the vegetation of the Sanctuary in its natural state. Although eradication of Prosopis is an expensive process, there are habitat improvement measures that have been undertaken from time to time to protect the sanctuary. However, if the attempts to remove the species are carried out in a larger scale, it would be beneficial in the long run.

The author Maithreyi Kamalanathan was a Documentation Intern at Care Earth and also a student of journalism and communication with a keen interest on documenting issues related to human-wildlife conflicts.